Steam generating unit



P 1964 N. s. BLODGETT 3,146,761 STEAM GENERATING UNIT Filed Jan. 23, 1962 3 Sheets-Sheet 1 INVENTOR. NORMAN S. BLODGETT v FIG.|I

Sept. 1, 1964 N. s. BLODGETT STEAM GENERATING UNIT v Filed Jan. 23, 1962 3 Sheets-Sheet 2 IN VEN TOR. NORMAN S. BLODGETT Sept. 1,1964

Filed Jan. 23, 1962 SUPERHEAT FINAL TEMPERATURE N. S. BLODGETT 3 Sheets-Sheet 3 LOAD I; m A E I [u I 5 a s A K.

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United States Patent 3,146,761 STEAM GENERATING UNIT Norman S. Blodgett, Westboro, Mass., assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts Filed Jan. 23, 1962, Ser. No. 168,043 7 Claims. (Cl. 122-479) This invention relates to a steam generating unit and more particularly to apparatus arranged to regulate the temperature of superheated steam in a boiler.

In the generation of steam for use in producing electricity by means of a turbine, it is necessary that the temperature of the steam reaching the turbine be maintained at a predetermined value with very little variation. If this is not done, the turbine will not operate at full efiiciency. Furthermore, in present-day steam generating units, the temperature of the steam is very close to the failure point of the metal in the superheater tube and strict regulation is necessary to assure that the temperature of the superheat does not go above the design value and cause tube failure. One of the methods commonly used for regulating the temperature of superheated steam is that of recirculation of gas through the main furnace from the back passes of the boiler. The effect of this recirculation is to cool the gases passing over the convection surfaces and also to increase the mass flow. Generally speaking, an increase in mass flow dominates the situation and the temperature goes up, despite the fact that the temperature of the gases passing over the superheater surfaces may be lower. Because of the fact that the recirculated gas is used in considerably greater amounts at low loads (where an increase in the transfer of heat to the superheated steam is more necessary), a number of problems are presented. First of all, since the normal products of combustion are smaller in amount at low loads, they do not mix as thoroughly with the recirculated gas which has been introduced into the furnace as at high loads. In addition, there are difficulties in maintaining ignition and in providing a suitable location for the introduction of the recirculated gas'. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a novel apparatus for the control of superheat by combining the effects of recirculation of gas and flame placement.

Another object of this invention is the provision of a steam generating unit making use of the recirculation of gas from the back passes to the main furnace in which gas temperature from side to side of the furnace are maintained substantially uniform and slagging is reduced to a minimum.

Another object of the instant invention is to provide a superheat control in which recirculated gas is very thoroughly mixed with the ordinary products of combustion before passage through the convection passes of the boiler.

It is a further object of the invention to provide a steam generating unit embodying a novel method of recirculating cooled products of combustion.

With these and other objects in View, as will be apparent to those skilled in the art, the invention resides in the combustion of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms as illustrated by the accompanying drawings in which:

FIG. 1 is a vertical longitudinal view of a steam generating unit embodying the principles of the present invention;

3,146,761 Patented Sept. 1, 1964 FIG. 2 is a schematic view of the apparatus showing a vertical section through burners at low load;

FIG. 3 is a schematic view of the apparatus showing a vertical section through gas openings at low load;

FIG. 4 is a schematic view of the apparatus showing a vertical section through burners at high load;

FIG. 5 is a schematic view of the apparatus showing a vertical section through gas openings at high load; and

FIG. 6 is a graph showing the variation of superheat temperature with changes in load.

Referring first to FIG. 1, which best shows the general features of the invention, the steam generating unit, indicated generally by the reference numeral 10, is shown as consisting of a furnace 11 and a boiler 12. The steam generating unit is mounted on a supporting structure 13. The furnace 11 is provided with a front wall 14, a rear wall 15, and side walls 16 defining a vertically-elongated combustion chamber 17. The furnace is of the type generally shown and described in the patent of Craig, No. 2,853,059, and is provided with a continuous slag bottom 18. The front wall 14 is provided with an abutment 19 which extends inwardly of the furnace; the abutment has an inclined upper surface 21 and a similarly inclined lower surface 22. The rear wall 15 is provided with a similar abutment 23 having an inclined upper surface 24 and an inclined lower surface 25. The inclined lower surfaces of the two abutments are provided with adjustable-vane, directional-flame, intertube burners 26 and 27 of the type shown in the patent to Craig, No. 2,759,460, which are connected in the usual way with sources of air and fuel. The'abutments 19 and 23 extend in opposition to each other across the combustion chamber and divide the combustion chamber into a high-temperature cell 28 and an upper portion 29. The boiler 12, which cooperates with the furnace 11 in the usual way, consists of a steam-and-water drum 31 from which radiate the various tubes of the boiler system. Large downcomer tubes (not shown) carry the water to the lower portion of the boiler and this flows upwardly through the system, particularly in water-wall tubes 20 which line the walls of the combustion chamber 17. Some of these water-wall tubes extend over the surfaces of the abutments 19 and 23. The directional-flame burners 26 and 27 are so arranged that the fuel and combustion air pass through gaps provided between certain of the water-wall tubes. Steam generated in the water-wall tubes 20 is introduced into thesteam-and-water wall drum 31. At the upper part of the steam-and-water drum are located tubes 32 leading to a low-temperature convection superheater 33. The outlet end of the low-temperature superheater 33 is connected to the inlet side of a radiant superheater 33 mounted on the front wall 14. The outlet side of this superheater is connected to the inlet side of a high-temperature superheater 34, which is partially radiant and partially convective and whose outlet is connected to the turbine (not shown) in the usual manner.

The steam generating unit is provided with a back pass 35 divided by a wall 36 formed of tubes into a forward pass 37 and a rearward pass 38. The low-temperature superheater 33 is located in the rearward pass 38 and a low-temperature reheater 39 is located in the forward pass 37. At the upper portion of the combustion chamber 17 alternate tubes of the rearward wall 15 are bent forwardly to form a nose 41 defining a horizontal upper pass 42. A high-temperature reheater 43 resides in this upper pass as does the terminal convection portion of the high-temperature superheater 34. The major portion of the hightemperature superheater 34, however, resides in the upper portion of the combustion chamber 17 and is formed into large pendant platens. The lower ends of these platens extend below the nose 41 into the upper portion 29 of the combustion chamber.

At the lower end of the rearward pass 38 are located dampers 44 which control the flow of gas therethrough; these dampers are connected to an actuating rod 45 so as to operate together and the rod 45 is connected to a linear actuator, such as a hydraulic cylinder 46. Hydraulic lines 47 and 48 lead to a main control 49 and are connected to the cylinder. The said main control 49 is similar to that shown and described in the co-pending patent application of Parmakian, Serial Number 774,074, filed November 14, 1958, and assigned to the assignee of the present case. In the lower end of the forward pass 47 are located dampers 51 which are movable together under the action of a rod 52 connected to a hydraulic cylinder 53 which operates under the control of hydraulic lines 54 and 55 connected to the main control 49.

The lower end of the back pass 35 of the steam generating unit is connected through an air heater 56 to a breaching 57 which leads to a stack (not shown). The outlet of the high-temperature reheater 43 is connected to a reheated steam header 58, while the outlet of the high-temperature superheater 34 is connected to a superheated steam header 59.

The lower surface of the abutment 19 is formed with a series of gas openings between pairs of the burners 26 in the manner shown in the copending application of Parmakian, Serial Number 95,675, filed March 14, 1961, and assigned to the assignee of the present case, now Patent No. 3,095,863, issued July 2, 1963. These openings are formed by displacing water-wall tubes to form passages between the burners. A similar series of openings is formed between pairs of the burners 27 on the undersurface of the abutment 23. A large conduit 61 is connected to the gas openings on the abutment 19, while a similar conduit 62 is connected to the openings on the underside of the abutment 23. The conduits 61 and 62 are connected to a single conduit 65 having mounted therein a regulating damper 66. The end of the conduit 65 which is away from the connection to the conduits 61 and 62 is connected through a duct 65) having a damper 7'11 to the outlet of a fan 67. The inlet of the fan 67 is connected by a conduit 68 having a damper 95 to a portion of the back pass, such as the portion of the steam generating unit following an economizer 96 and lying between the dampers 44 and 51 on the one hand and the air heater on the other hand.

The damper 66 is pivotally mounted and is actuated through rods 65 and 71, the latter rods being connected to a hydraulic cylinder '72, whose linear position is determined by the fluid pressure hydraulic lines '73 and 74 connected to the main control 49. The main control is connected through a line 75 to a temperature-indicating device such as the thermocouple '76 located in the superheated steam header 59. The main control 49 is connected in a similar manner by a line 77 to a temperatureindicating device such as the thermocouple 78 lying in the reheated steam header 58.

As has been stated above, the burners 26 are alternately located with recirculated gas openings. Both the burner openings and the recirculated gas openings are provided by bending back certain of the water-wall tubes 20. This is accomplished by bending four groups of three tubes back into a plane at a right angle to the surface of the front wall. A small number of tubes are left in the wall between the burners and gas openings. A typical burner is provided with a main fuel gun which extends centrally thereof and which is connected to a conduit from which it receives powdered coal and primary air. A gas gun is located centrally of the main fuel gun for maintaining ignition and an igniter is located in the burner closely adjacent the main fuel gun, the igniter being of the elecric spark variety using gas as its fuel. Arranged along the burner above the gun are vanes whose inclination to the horizontal is adjustable by means of a control rod. In the lower part of the burner, vanes 87 are controlled by a rod 86. Air arrives at the burner through a duct 89.

A typical recirculated gas opening is provided with water tubes 20 which have been bent back to form a generally lozenge-shaped opening which is lined with refractory. A duct 92 extends inwardly to each gas opening on one side of the furnace from the main conduit 61, while the recirculated gas openings on the other side of the burners are connected to the main conduit 62, both of these lastnamed conduits extending parallel to the side and rear walls of the furnace outside of the line of air plenum chambers associated with the main fuel burners. Associated with each recirculated gas opening is a series of vanes which are pivotally mounted for rotation about a horizontal axis and controllable by a rod.

Connected to the duct 60 between the damper 7G and the duct 65 is a duct 97 which is provided with a damper 98. This damper is connected to a hydraulic cylinder 99 which is connected by lines 101 and 102 to the main control 49. The other end of the duct 97 is connected to the hollow interior of the nose 41. The nose is lined with refractory 163 with the exception of a portion of the underside where there is an opening 164; the fact that the nose 41 is formed of alternate tubes from the rear wall 15 means that substantial spaces exist between the tubes crossing the opening 184.

A gas duct 165 is connected at one end to the duct 68 between the damper and the fan 67; the other end is connected to the back pass at a location upstream of the economizer 96 at an opening 106 in the rearward pass 38 under the low-temperature superheater 33. The duct 165 is provided with a damper 167 controllable by a cylinder 168 whose condition is determined by signals received from the main control 49 through lines 109 and 111. The damper '71) in the duct 66 is regulated by a cylinder 112 which is connected to the main control 49 by lines 113 and 114.

The upper portion of the low-temperature superheater 33 is connected to the radiant superheater 30 by a pipe 115, and in this pipe is located a spray-type desuperheater 116 which is connected for regulation to the main control 49 by a line 117. The vanes of the burners 26 are connected by the rod 86 to a cylinder 118 which is regulated by the main control 49 through the medium of lines 119 and 121. The vanes of the burners 27 are operated by a cylinder 122 which receives control signals from the main control 49 through connecting lines 123 and 124. The damper 95 in the duct 68 is adjusted by a cylinder 125 which is connected to the main control 49 by lines 126 and 127. The arrangement of burners 26 and 27 with their vanes, their cylinders 118 and 122, and the main control 49 provide for the flame placement operation shown in the patent of Miller, No. 2,947,289.

The operation of the invention will now be readily understood in view of the above description. Fuel and air leave the burners 26 and 27 and enter the high-temperature cell 23 of the combustion chamber 17. The cell 23 exists at high temperature because of the restricted exit between the abutments 19 and 23 which causes the temperature in the portion 28 to reach a high value. Extreme turbulence takes place because of the opposed relationship of the burners and because of the presence in the cell of a pool of molten slag. A considerable portion of the combustion takes place in the cell 28 and, because of the high temperature, most of the formation of slag takes place there and is persuaded to stay there. The products of combustion pass upwardly through the combustion chamber 17 and a certain amount of combustion takes place in the upper portion 29. The gases leave the upper portion of the combustion chamber, flow through the horizontal pass 42, and then flow downwardly through the back pass 35. The amounts of gas flowing through the forward portion 37 and the rearward portion 38 of the back pass are determined by the settings of the dampers 51 and 44. Gases pass through the air heater 56 into the breaching 57 and then flow into the stack. Heat absorbed by the water-walls of the furnace causes the generation of steam; this steam enters the steam-and-water drum and, after suitable cleaning in the usual manner, passes downwardly into the lower portion of the low-temperature superheater 33, passing upwardly therethrough in a direction counter to the flow of gases. After being superheated in this manner to a certain degree, the steam then passes into the pipe 115 where its temperature may be reduced by the desuperheater 116. At the other end of the pipe the steam enters the radiant superheater 3d. From there the steam flows to the hightemperature superheater 34, entering the portion thereof adjacent the front Wall 14 of the furnace 11. The steam in the high-temperature superheater is first subjected to a considerable amount of radiation because of the fact that a large part of this superheater is in the form of platens which extend deeply into the main combustion chamber 17. The steam is then subjected to a small degree of convection superheating, passes into the superheated steam header 59, and flows from there to the highpressure section of the turbine (not shown).

After passing through the high-pressure section of the turbine the steam returns to the boiler for reheating. It enters the low-temperature reheater 39 in its lower por tion and passes upwardly therethrough in counterflow to the flow of gases in the forward portion 37 of the back pass. Eventually, the steam reaches the high-temperature section 43 of the reheater, passes forwardly into the horizontal pass 42 in counterfiow to the flow of gases therethrough, and eventually enters the reheated steam header S8 for passage through the low-pressure section of the turbine.

The relationship between the superheat and the reheat temperatures can be controlled in the usual way by regulating the portions of the total amount of products of combustion which pass through the forward portion 37 or the rearward portion 38 of the backpass. The regulation of dampers 44 and 51 accomplishes this purpose. It is not possible, however, to accomplish control of the absolute value of these variables by this means alone. By properly setting the damper 70 a certain amount of the products of combustion are carried back into the furnace. These products leave the back passes of the furnace through the conduits 68 and 165 under the impetus of the fan 67 and enter the conduit 69. They are then divided between the conduits 97 and 65 and enter the furnace either through the opening 1% in the nose 41 or through the gas openings 79 on the lower surfaces of the abutments l9 and 23. The introduction of large amounts of gas which have been recirculated in this manner has the effect of increasing the mass flow of gases over the heat exchange unit in the boiler. With respect to the convection sections of a boiler, an increase in mass flow (even though this increase is brought about by the introduction of lower temperature gases), results in a greater amount of heat transfer and at a given load would result in a raising of the temperature of steam flowing within the convection elements. Ordinarily, of course, the recirculation of gas would be reserved only for those portions of load in which substantial increases of heat transfer to the steam are necessary. Referring to FIG. 6 which shows the relationship of the superheat temperature throughout various loads and under various types of furnace operation, let us suppose that the dampers 44, 51, and '76 are such that the preselected value of superheat is met at full load. If the load is lowered, that is to say, if the demands of the turbine for quantities of steam is reduced, the firing rates of the burners 26 and 27 are also reduced. This means that there is less gas flow and lower gas temperatures throughout the furnace and boiler. The superheat will tend to drop, and this will be indicated by the temperature-indicating thermocouple 76 which will transmit signals through the line 75 to the main control 49. Throttling the dampers together will not bring about a sufficient increase in heat transfer to reheat and superheat, so that it is necessary to make use of other means. The main control 49 senses that damper control will do nothing to maintain the reheat and superheat temperatures, so it operates through the hydraulic lines 113 and 114 to actuate the hydraulic cylinder 112. The cylinder operates through the connecting rods to reset the damper 70. The resetting in this case will be in the direction of opening the damper and causing a greater amount of recirculated gas to flow into the furnace. The increase in mass flow thus accomplished will maintain both the reheat and the superheat temperatures at the selected value. If one or the other of the two temperatures varies from the preselected value, the main control will operate through hydraulic lines 47, 48, 54, and 55 to move the hydraulic cylinders 46 and 53 to reset the dampers 44 and $1, respectively. For instance, if reheat is adequate but superheat is low, it might be necessary to open the damper 44 slightly while throttling the damper 51.

The introduction of the recirculated gas into the furnace by means of the gas openings between the burners has a number of unobvious advantages which make it particularly effective with the type of furnace shown. Gases leaving the cell 28 of the combustion chamber are restricted to the area between the abutments 19 and 23 and flow into the upper portion 29 of the combustion chamber along lines which are distinctly separated from the forward wall 14 and the rearward wall 15 of the furnace. The main effect of this is that any slag particles which reside in the gas flow have a good opportunity to cool off and form hard particles before they strike any heat exchange surface. The tendency of this type of furnace is to keep the gases away from the front and rear walls until such cooling takes place. This separation or stratification of the gases away from the walls cooperates with the function of almost complete combustion in the cell 28 to produce a very clean furnace. It has the effect also of providing for a greater uniformity of furnace temperature from side to side of the furnace and makes possible a smaller necessary furnace size. Now, when the recirculated gas is introduced through the gas openings in the lower surface of the abutments, the normal position of the vanes in these openings would be upward, whereas the vanes of burners will be inclined downwardly into the high-temperature cell 28. The recirculated gas thus introduced through the opening 79 passes outwardly around the noses 19 and 23 and further accentuates the separation of the main body of the products of combustion from these walls. It might be said that the recirculated gases form an insulating blanket be tween the main products of combustion and the front and rear walls, thus providing the benefit of less slagging and fouling of the water walls. At the upper portion of the furnace the reversal of gas direction by the nose 41 and horizontal pass 42 produces sufiicient mixing of the recirculated gases and the main product of combustion to accomplish the increases in mass flow and thorough mixing of gases which are necessary to good control of reheat and superheat. As has been stated, in the high-temperature cell 28 combustion takes place almost entirely below the level of the abutments 19 and 23. Furthermore, the Stratification of the recirculated gases adjacent the walls 14 and 15 assures that these gases will mix very little with the main products of combustion until they reach a point high in the combustion chamber. This means that combustion has a proper opportunity to take place with the proper mixture of fuel and air long before the recirculated gases are mixed with them.

While the damper 70 in the duct regulates the total amount of gas recirculated to the furnace, the settings of the damper 66 in the duct and the damper 98 in the duct 97 determine the relative quantities of these gases which will pass through the gas openings between the burners 26 and 27 and through the opening 104 in the nose 41. The main control 49 operating through the control lines 73, 74, 101 and 102 assures that the settings of the dampers 66 and 98 are at all times oppositely complementary; that is to say, when the damper 66 is open, the damper M3 will be closed, and vice versa. Furthermore, at intermediate positions of one damper, the other damper will be in such a position that the total recirculated gas will be properly divided between the burner level and the nose level.

Means is also provided for regulating the absolute temperature of the recirculated gases. The operation of the damper 95 in the duct 68 and the damper 167 in the duct 105 determines the proportions of the gases will be hot (coming from the opening 106 above the economizer 96) and will be cool (coming from a portion of the apparatus below the economizer 96). Here again, the operation of the dampers 95 and 197 are oppositely complementary, so that, when the damper 95 is wide open, the damper 107 will be closed, and vice versa; furthermore, at intermediate positions the settings of the dampers will be such that adequate cross-sectional flow capacity will be available in the two ducts 68 and 105 to satisfy the setting of the main control damper 76 in the recirculated gas duct 60.

The operation of the desuperheater 116 by the main control 49 through the line 117 is well known and in the present apparatus is used to maintain the temperature of superheat at design value when the steam value would otherwise be higher than the predetermined design value, due to inaccuracies in other parts of the equipment.

In general, therefore, the apparatus provides for superheat control by adjusting the flame position from the front to the rear of the furnace by regulating the total amount of gas recirculated to the furnace, by regulating the relative amounts of recirculated gas passing into the furnace at burner level and at the nose, and by regulating the temperature of the recirculated gas. Furthermore, regulation of superheat control takes place by use of the desuperheater 116 and by the proper use of the dampers 44 and 51.

At low load, which is the situation shown in FIGS. 2 and 3, the vane settings of the burners is such as to cause the flame to be located adjacent the front wall of the furnace, which has the effect of giving high superheat by decreasing the residence time of the gases. At the same time, the amount of gas recirculated will be large, that is to say, the setting of the damper '70 will be at its open position. Furthermore, the damper 98 will be open and the damper 66 closed to cause most of the gas to be introduced through the opening 104 in the nose 41.

As can be seen in FIG. 3, a small quantity of gas will be permitted to pass into the furnace at the burner level to provide for blanketing of the front and rear walls of the furnace. The large amount of recirculated gas has the effect of maintaining the temperature of superheat at a constant value as does the introduction of this gas at the top of the furnace rather than at the bottom. It should be noted also that the positioning of the flame adjacent the front wall and adjacent the radiant superheater 30 tends to make the superheat temperature high. Furthermore, by opening the damper 107 and closing the damper 95 the recirculated gas will be at a higher temperature, which means that, when the gas passes into the convection passes of the furnace, the effect will be to keep the superheat up to design value.

At high load, the situation shown in FIGS. 4 and 5 will prevail. As shown in FIG. 4, the burner vane settings will be such that the flame will be positioned near the rear wall of the furnace, so that the gas from the flame will pass under the nose 41 and then flow into the convection pasess. It will, therefore, be positioned as far from the radiant superheater as possible and will have a longer residence time in the furnace so that the gas temperature will be lower when it reaches the convection pass of the furnace. This is in absolute contrast to the situation shown in FIG. 2, where the flame takes the shortest route from the burners to the convection passes and, therefore, has the shortest residence time in the furnace whereby the gas temperature will be high when it reaches the convection passes. At high load the total amount of recirculated gas will be low, accomplished by a partial closure of the damper in the duct 60. Furthermore, the division of the gases between the ducts 97 and the duct 65 will be such that most of the gases will be introduced into the furnace at burner level; for this purpose the damper 58 will be closed and the damper 66 open. A small amount of gas will be permitted to flow through the opening 104 to cool the tubes at that point and to protect the interior of the nose 41. As is evident in FIG. 5, the larger proportion of the recirculated gases at high load will be used to protect the front and rear walls of the furnace, the greater amount of fuel being burned at high load requiring more adequate protection of the walls from slag. At the same time, the introduction of these recirculated gases at the bottom of the furnace assures that the temperature of gases reaching the convection passes will be lower. At the same time, the settings of the dampers and 107 are such that the damper 107 would be closed, while the damper 95 will be open, so that those recirculated gases which are introduced into the furnace at high load are at the lowest possible temperature. All of these factors, i.e., flame placement, amount of gas recirculated, position of gas introduction into the furnace, and temperature of recirculated gas are selected so that at high load setting the characteristic curve of superheat versus load is low. As shown in FIG. 6, the boiler is designed so that the characteristic curve with the high load setting will provide the desired design temperature at load; at loads over 100% the desuperheater 116 will reduce the superheat temperatuer to the desired value.

Although in the above description it has been intimated that the high load setting would be used at loads above 100% load, and that at loads below 100% the low load setting would be used (thus requiring the desuperheating of a considerable amount of steam at intermediate loads), it will be understood that the main control 49 may operate the dampers through the hydraulic cylinders in such a manner that no desuperheating would be needed between 100% load and 50% load. The characteristic curve would be any one of a family between the two curves shown in FIG. 6. For instance, at 75% load, there is a curve having the same sloping characteristic as the two curves shown in FIG. 6, which curve will pass through the desired superheat temperature line at 75% load. The main control 49 will select the proper settings of the elements to provide the particular characteristic curve needed for a particular load to obtain such a crossing. In other words, as the load is dropped from 100% load to 50% load, the settings of the vanes of the burners 26 and 27 will be contiuously adjusted from the positions which cause the flame to be positioned near the front wall of the furnace to a position where the flame rises along the rear wall, the flame at intermediate loads occupying intermediate positions between the front and rear walls. In the same way, as the load goes from 100% load to 50% load, the damper 70 (which controls the total amount of gas recirculated) will be continuously adjusted from a closed position at 100% load to an open position at 50% load; at intermediate loads the damper 70 will occupy intermediate positions between open and closed. In the same way, as the load is moved from 100% load to 50% load, the settings of the dampers 66 and 98 Will be moved from a condition in which the damper 66 is wide open and the damper 98 is closed (at 100% load) to a condition where the damper 66 is closed and the damper Q8 is wide open (at 50% load); at intermediate loads, the settings of the dampers will be at intermediate positions proportionate to the particular load. In a similar manner, as the load is adjusted from 100% load to 50% load, the dampers 95 and 107 (which regulate the temperature of the recirculated gas) will be adjusted from a position where the damper 95 is wide open and the damper 107 is closed (at high load) to a condition where the damper 95 is closed and the damper 107 is open (at low load); at intermediate loads, the dampers will be at intermediate positions to give intermediate temperatures of recirculated gas commensurate with the particular load at any given time. This manner of adjusting the dampers contiuously as one moves from high load to low load is indicated by the heavy line in FIG. 6.

Certain minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is new and desired to secure by Letters Patent is:

1. A steam generating unit, comprising front, rear, and side walls defining a vertically-elongated combustion chamber, a convection superheater section located in the upper part of the furnace, opposed abutments providing a high-temperature cell at the lower end of the furnace, fuel-burning means located in the cell for producing an upwardly-directed flame, a first means associated with the fuel-burning means positioning the said flame adjacent the front wall at low load and adjacent the rear wall at high load, gas recirculation means for gathering gases from two locations substantially spaced from one another in the direction of gas How and introducing the gas so gathered into the furnace at upper and lower positions located close to and far from the convection superheater section, a second means associated with the gas recirculating means regulating the total amount of gas introduced into the furnace, a third means associated with the gas recirculation means determining the relative proportions of the gas introduced into the furnace at the upper and lower positions, a fourth means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations, a load indicator, and a main control connected to the load indicator and connected to the first, second, third, and fourth means to maintain the temperature of superheat at a predetermined constant value by regulating the said first, second, third, and fourth means in accordance with a signal from the load indicator.

2. A steam generating unit, comprising front, rear, and side walls defining a vertically-elongated combustion chamber, a convection superheater section located in the upper part of the furnace, opposed abutments providing a high-temperature cell at the lower end of the furnace, fuel-burning means located in the cell for producing an upwardly-directed flame, a first means associated with the fuel-burning means positioning the said flame adjacent the front wall at low load and adjacent the rear wall at high load, gas recirculation means for gathering gases from two locations substantially spaced from one another in the direction of gas flow and introducing the gas so gathered into the furnace at upper and lower positions located close to and far from the convection superheater section, a second means associated with the gas recirculating means regulating the total amount of gas introduced into the furnace, a third means associated with the gas recirculation means determining the relative proportions of the gas introduced into the furnace at the upper and lower positions, a device measuring the temperature of superheat and transmitting a signal indicative thereof, a fourth means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations, a load indicator, and a main control connected to the load indicator and connected to the said device to receive the said signal and connected to the first, second, third,

and fourth means to transmit to them corrective signals to maintain the temperature of superheat at a predetermined constant value by regulating the said first, second, third, and fourth means in accordance with a signal from the load indicator.

3. A steam generating unit, comprising a verticallyelongated combustion chamber, a convection superheater section located so as to receive gas from the upper part of the furnace and located closer to one of two opposed walls of the chamber than the other, fuel-burning means located at the lower par-t of the furnace for producing an upwardly-directed flame, a first means associated with the fuel-burning means adjusting the said flame to positions relative to the said opposed walls from adjacent the said other wall at low load to adjacent the said one wall at high load, gas recirculation means for gathering gases from two locations substantially spaced from one another in the direction of gas flow and introducing the gas so gathered into the furnace at upper and lower positions located close to and far from the convection superheater action, a second means associated with the gas recirculating means regulating the total amount of gas introduced into the furnace from a large amount at low load to a small amount at high load, a third means associated with the gas recirculation means deterrmining the relative proportions of the gas introduced into the furnace at the upper and lower positions from a high proportion to the upper position at low load to a high proportion to the lower position at high load, a fourth means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations from a high proportion from the upstream location at low load to a high proportion from the downstream location at high load, a load indicator, and a main control connected to the load indicator and connected to the first, second, third, and fourth means to maintain the temperature of superheated steam at a predetermined constant value by regulating the said first, second, third, and fourth means in accordance with a signal from the load indicator.

4. A steam generating unit, comprising a verticallyelongated combustion chamber, a convection superheater section located so as to receive gas from the upper part of the furnace and located closer to one of two opposed Walls of the chamber than to the other, fuel-burning means located at the lower part of the furnace for producing an upwardly-directed flame, a first means associated with the fuel-burning means adjusting the said flame to positions relative to the said opposed walls from a position adjacent the said other wall at low load to a position adjacent the said one wall at high load, gas recirculation means for gathering gases from two locations substantially spaced from one another in the direction of gas flow and introducing the gas so gathered into the furnace at upper and lower positions located close to and far from the convection superheater section, a second means associated with the gas recirculation means determining the relative proportions of the gas introduced into the furnace at the upper and lower positions from a high proportion to the upper position at low load to a high proportion to the lower position at high load, a third means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations from a high proportion from the upstream location at low load to a high proportion from the downstream location at high load, a load indicator, and a main control connected to the load indicator and connected to the first, second, and third means to maintain the temperature of superheated steam at a predetermined constant value by regulating the said first, second, and third means in accordance with a signal from the load indicator.

5. A steam generating unit, comprising a verticallyelongated combustion chamber, a convection superheater section located so as to receive gas from the upper part of the furnace and located closer to one of two opposed walls of the chamber than to the other, fuel-burning means located at the lower part of the furnace for producing an upwardly-directed flame, gas recirculation means for gathering gases from two locations substantially spaced from one another in the direction of gas flow and introducing the gas so gathered into the furnace at upper and lower positions located close to and far from the convection superheater section, a first means associated with the gas recirculating means regulating the total amount of gas introduced into the furnace from a large amount at low load to a small amount at high load, a second means associated with the gas recirculation means determining the relative proportions of the gas introduced into the furnace at the upper and lower positions from a high proportion to the upper position at low load to a high proportion to the lower position at high load, a third means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations from a high proportion from the upstream location at low load to a high proportion from the downstream location at high load, a load indicator, and a main control connected to the load indicator and connected to the first, second, and third means to maintain the temperature of superheated steam at a predetermined constant value by regulating the said first, second, and third means in accordance with a signal from the load indicator.

6. A steam generating unit, comprising a verticallyelongated combustion chamber, a convection superheater section located so as to receive gas from the upper part of the furnace and located closer to one of two opposed walls of the chamber than to the other, fuel-burning means located at the lower part of the furnace for producing an upwardly-directed flame, a first means associated with the fuel-burning means adjusting the said flame to positions relative to the said opposed walls from a position adjacent the said other wall at low load to a position adjacent the said one wall at high load, gas recirculation means for gathering relatively cool gases from two locations substantially spaced from one another in the direction of gas flow and introducing the gas so gathered into the furnace at positions close to and far from the convection superheater section, a second means associated with the gas recirculating means regulating the total amount of gas introduced into the furnace from a large amount at low load to a small amount at high load, a third means associated with the gas recirculation means determining the relative proportions of the gas gathered from the said two locations from a high proportion from the upstream location at low load to a high proportion from the downstream location at high load, a load indicator, and a main control connected to the load indicator and connected to the first, second, and third means maintaining the temperature of superheated steam at a predetermined constant value by regulating the said first, second, and third means in accordance with a signal from the load indicator.

7. Apparatus for controlling superheat in a steam generating unit, comprising walls defining a vertically-elongated furnace, a back pass through which the products of combustion travel after they leave the furnace, abutments on opposed walls, each abutment having inclined upper and lower surfaces intersecting inwardly of the furnace walls to form a restriction in the furnace, the furnace, being provided with a slagging bottom, directional-flame burners having adjustable vanes located on the said lower inclined surfaces of the abutments, a series of first openings into the furnace located on the lower surfaces of the abutments between the burners, the openings having upwardly inclined vanes, a first conduit connected at oneend to the first openings, a second opening located in the upper part of the furnace, a second conduit connected at one end to the second opening, a fan whose outlet is connected to the other ends of the first and second conduits, a third conduit connecting the inlet of the fan to a lower position in the said back pass, a fourth conduit connecting the inlet of the fan to a higher position in the back pass, a damper in each of the conduits, a convection superheater located downstream of both of the said openings, an indicator associated with the superheater producing a signal representative of the degree of superheat, actuators for determining the setting of the said dampers, a load indicator, and control means connected to the load indicator and receiving the signals from the indicator and transmitting signals to the actuators to regulate the dampers, thus increasing and decreasing the fiow of gas through the conduits from the said positions in the back pass to the openings in the furnace, thus increasing and decreasing the heat transfer to the superheater to maintain the superheat at a predetermined value by regulating the said control means in accordance with a signal from the load indicator.

References Cited in the file of this patent UNITED STATES PATENTS 2,897,797 Koch Aug. 4, 1959 2,947,289 Miller Aug. 2, 1960 2,966,897 Smith Jan. 3, 1961 FOREIGN PATENTS 549,492 Belgium Jan. 12, 1957 793,048 Great Britain Apr. 9, 1953 OTHER REFERENCES Combustion of June 1959, published by Combustion Publishing Company, New York, article on pages 38 to 44 relied upon. 

1. A STEAM GENERATING UNIT, COMPRISING FRONT, REAR, AND SIDE WALLS DEFINING A VERTICALLY-ELONGATED COMBUSTION CHAMBER, A CONVECTION SUPERHEATER SECTION LOCATED IN THE UPPER PART OF THE FURNACE, OPPOSED ABUTEMENTS PROVIDING A HIGH-TEMPERATURE CELL AT THE LOWER END OF THE FURNACE, FUEL-BURNING MEANS LOCATED IN THE CELL FOR PRODUCING AN UPWARDLY DIRECTED FLAME, A FIRST MEANS ASSOCIATED WITH THE FUEL-BURNING MEANS POSITIONING THE SAID FLAME ADJACENT THE FRONT WALL AT LOW AND ADJACENT THE REAR WALL AT HIGH LOAD, GAS RECIRCULATION MEANS FOR GATHERING GASES FROM TWO LOCATIONS SUBSTANTIALLY SPACED FROM ONE ANOTHER IN THE DIRECTION OF GAS FLOW AND INTRODUCING THE GAS SO GATHERED INTO THE FURNACE AT UPPER AND LOWER POSITIONS LOCATED CLOSE TO AND FAR FROM THE CONVECTION SUPERHEATER SECTION, A SECOND MEANS ASSOCIATED WITH THE GAS RECIRCULATING MEANS REGULATING THE TOTAL AMOUNT OF GAS INTRODUCED INTO THE FURNACE, A THIRD MEANS ASSOCIATED WITH THE GAS RECIRCULATION MEANS DETERMINING THE RELATIVE PROPORTIONS OF THE GAS INTRODUCED INTO THE FURNACE AT THE UPPER AND LOWER POSITIONS, A FOURTH MEANS ASSOCIATED WITH 